Nonequilibrium dynamics of hot carriers and hot phonons in CdSe and GaAs

The dynamics of hot carriers has been investigated extensively for III-V compound semiconductors in the past but not as well for II-VI semiconductors. In this paper, we calculate the hot electron and hole energy-loss rates (ELR’s) in CdSe taking into account all relevant phonon emission processes. Interband and intraband scattering in the two uppermost valence bands (A and B) in CdSe are included. We make a detailed comparison of these ELR’s with those for GaAs. We then report our experimental measurements of time- and energy-resolved luminescence in CdSe performed using upconversion luminescence spectroscopy with a time resolution of 2.5 ps. Using these results, we obtain the hot-carrier cooling rates in CdSe in the initial time domain up to 50 ps following photoexcitation at t=0 ps. By varying the excited carrier density (${\mathit{n}}_0$) within the range ${10}^{17}$–${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, we find that the hot-carrier cooling behavior has a noticeable dependence on the excited carrier density ${\mathit{n}}_0$, the cooling getting slower as ${\mathit{n}}_0$ increases. We present a detailed comparison of the experimental cooling results with our calculations of the ELR’s. The measured cooling rates are much smaller (by over two orders of magnitude) than expected in this theory. The slow cooling can be explained within the hot-phonon theory which takes into account the coupled dynamics of hot carriers and nonequilibrium optical phonons. Using this, we deduce that the lattice dynamical lifetime of the optical phonons in CdSe is about 5 ps (at 8 K, ${\mathit{n}}_0$=2×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$). We find that the energy-loss rates (ELR’s) are initially dominated by the Fröhlich interaction of the electrons and holes but later, by transverse optical phonon emission by holes, as the carriers cool. It is shown that the reduction in the ELR’s as a result of the hot-phonon effects is nearly ten times larger in CdSe than in GaAs. We also study the time dynamics of the nonequilibrium optical phonon occupancies in both CdSe and GaAs.